Internet Engineering Task Force (IETF) J. Klensin
Request for Comments: 6530 Y. Ko
Obsoletes: 4952, 5504, 5825 February 2012
Category: Standards Track
ISSN: 2070-1721
Overview and Framework for Internationalized Email
Abstract
Full use of electronic mail throughout the world requires that
(subject to other constraints) people be able to use close variations
on their own names (written correctly in their own languages and
scripts) as mailbox names in email addresses. This document
introduces a series of specifications that define mechanisms and
protocol extensions needed to fully support internationalized email
addresses. These changes include an SMTP extension and extension of
email header syntax to accommodate UTF-8 data. The document set also
includes discussion of key assumptions and issues in deploying fully
internationalized email. This document is a replacement for RFC4952; it reflects additional issues identified since that document
was published.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6530.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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RFC 6530 Internationalized Email Framework February 2012
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
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RFC 6530 Internationalized Email Framework February 20121. Introduction
In order to use internationalized email addresses, it is necessary to
internationalize both the domain part and the local part of email
addresses. The domain part of email addresses is already
internationalized [RFC5890], while the local part is not. Without
the extensions specified in this document, the mailbox name is
restricted to a subset of 7-bit ASCII [RFC5321]. Though MIME
[RFC2045] enables the transport of non-ASCII data, it does not
provide a mechanism for internationalized email addresses. In RFC2047 [RFC2047], MIME defines an encoding mechanism for some specific
message header fields to accommodate non-ASCII data. However, it
does not permit the use of email addresses that include non-ASCII
characters. Without the extensions defined here, or some equivalent
set, the only way to incorporate non-ASCII characters in any part of
email addresses is to use RFC 2047 coding to embed them in what RFC5322 [RFC5322] calls the "display name" (known as a "name phrase" or
by other terms elsewhere) of the relevant header fields. Information
coded into the display name is invisible in the message envelope and,
for many purposes, is not part of the address at all.
This document is a replacement for RFC 4952 [RFC4952]; it reflects
additional issues, shared terminology, and some architectural changes
identified since that document was published. It obsoletes that
document. The experimental descriptions of in-transit downgrading
[RFC5504] [RFC5825] are now irrelevant and no longer needed due to
the changes discussed in Section 12. The RFC Editor is requested to
move all three of those documents to Historic.
The pronouns "he" and "she" are used interchangeably to indicate a
human of indeterminate gender.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119
[RFC2119].
2. Role of This Specification
This document presents the overview and framework for an approach to
the next stage of email internationalization. This new stage
requires not only internationalization of addresses and header
fields, but also associated transport and delivery models. A prior
version of this specification, RFC 4952 [RFC4952], also provided an
introduction to a series of experimental protocols [RFC5335]
[RFC5336] [RFC5337] [RFC5504] [RFC5721] [RFC5738] [RFC5825]. This
revised form provides overview and conceptual information for the
Standards Track successors of a subset of those protocols. Details
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RFC 6530 Internationalized Email Framework February 2012
of the documents and the relationships among them appear in Section 5
and a discussion of what was learned from the experimental protocols
and their implementations appears in Section 6.
Taken together, these specifications provide the details for a way to
implement and support internationalized email. The document itself
describes how the various elements of email internationalization fit
together and the relationships among the primary specifications
associated with message transport, header formats, and handling.
This document, and others that comprise the collection described
above, assume a reasonable familiarity with the basic Internet
electronic mail specifications and terminology [RFC5321] [RFC5322]
and the MIME [RFC2045] and 8BITMIME [RFC6152] ones as well. While
not strictly required to implement this specification, a general
familiarity with the terminology and functions of IDNA [RFC5890]
[RFC5891] [RFC5892] [RFC5893] [RFC5894] are also assumed.
3. Problem Statement
Internationalizing Domain Names in Applications (IDNA) [RFC5890]
permits internationalized domain names, but deployment has not yet
reached most users. One of the reasons for this is that we do not
yet have fully internationalized naming schemes. Domain names are
just one of the various names and identifiers that are required to be
internationalized. In many contexts, until more of those identifiers
are internationalized, internationalized domain names alone have
little value.
Email addresses are prime examples of why it is not good enough to
just internationalize the domain name. As most observers have
learned from experience, users strongly prefer email addresses that
resemble names or initials to those involving seemingly meaningless
strings of letters or numbers. Unless the entire email address can
use familiar characters and formats, users will perceive email as
being culturally unfriendly. If the names and initials used in email
addresses can be expressed in the native languages and writing
systems of the users, the Internet will be perceived as more natural,
especially by those whose native language is not written in a subset
of a Roman-derived script.
Internationalization of email addresses is not merely a matter of
changing the SMTP envelope; or of modifying the "From:", "To:", and
"Cc:" header fields; or of permitting upgraded Mail User Agents
(MUAs) to decode a special coding and respond by displaying local
characters. To be perceived as usable, the addresses must be
internationalized and handled consistently in all of the contexts in
which they occur. This requirement has far-reaching implications:
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RFC 6530 Internationalized Email Framework February 2012
collections of patches and workarounds are not adequate. Even if
they were adequate, a workaround-based approach may result in an
assortment of implementations with different sets of patches and
workarounds having been applied with consequent user confusion about
what is actually usable and supported. Instead, we need to build a
fully internationalized email environment, focusing on permitting
efficient communication among those who share a language and writing
system. That, in turn, implies changes to the mail header
environment to permit those header fields that are appropriately
internationalized to utilize the full range of Unicode characters, an
SMTP extension to permit UTF-8 [RFC3629] [RFC5198] mail addressing
and delivery of those extended header fields, support for
internationalization of delivery and service notifications [RFC3461]
[RFC3464], and (finally) a requirement for support of the 8BITMIME
SMTP extension [RFC6152] so that all of these can be transported
through the mail system without having to overcome the limitation
that header fields do not have content-transfer-encodings.
4. Terminology
This document assumes a reasonable understanding of the protocols and
terminology of the core email standards as documented in RFC 5321
[RFC5321] and RFC 5322 [RFC5322].
4.1. Mail User and Mail Transfer Agents
Much of the description in this document depends on the abstractions
of "Mail Transfer Agent" ("MTA") and "Mail User Agent" ("MUA").
However, it is important to understand that those terms and the
underlying concepts postdate the design of the Internet's email
architecture and the application of the "protocols on the wire"
principle to it. That email architecture, as it has evolved, and
that "on the wire" principle have prevented any strong and
standardized distinctions about how MTAs and MUAs interact on a given
origin or destination host (or even whether they are separate).
However, the term "final delivery MTA" is used in this document in a
fashion equivalent to the term "delivery system" or "final delivery
system" of RFC 5321. This is the SMTP server that controls the
format of the local parts of addresses and is permitted to inspect
and interpret them. It receives messages from the network for
delivery to mailboxes or for other local processing, including any
forwarding or aliasing that changes envelope addresses, rather than
relaying. From the perspective of the network, any local delivery
arrangements such as saving to a message store, handoff to specific
message delivery programs or agents, and mechanisms for retrieving
messages are all "behind" the final delivery MTA and hence are not
part of the SMTP transport or delivery process.
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RFC 6530 Internationalized Email Framework February 20124.2. Address Character Sets
In this document, an address is "all-ASCII", or just an "ASCII
address", if every character in the address is in the ASCII character
repertoire [ASCII]; an address is "non-ASCII", or an "i18n-address",
if any character is not in the ASCII character repertoire. Such
addresses MAY be restricted in other ways, but those restrictions are
not relevant to this definition. The term "all-ASCII" is also
applied to other protocol elements when the distinction is important,
with "non-ASCII" or "internationalized" as its opposite.
The umbrella term to describe the email address internationalization
specified by this document and its companion documents is "SMTPUTF8".
For example, an address permitted by this specification is referred
to as a "SMTPUTF8 (compliant) address".
Please note that, according to the definitions given here, the set of
all "all-ASCII" addresses and the set of all "non-ASCII" addresses
are mutually exclusive. The set of all addresses permitted when
SMTPUTF8 appears is the union of these two sets.
4.3. User Types
An "ASCII user" (i) exclusively uses email addresses that contain
ASCII characters only, and (ii) cannot generate recipient addresses
that contain non-ASCII characters.
An "internationalized email user" has one or more non-ASCII email
addresses, or is able to generate recipient addresses that contain
non-ASCII characters. Such a user may have ASCII addresses too; if
the user has more than one email account and a corresponding address,
or more than one alias for the same address, he or she has some
method to choose which address to use on outgoing email. Note that
under this definition, it is not possible to tell from an ASCII
address if the owner of that address is an internationalized email
user or not. (A non-ASCII address implies a belief that the owner of
that address is an internationalized email user.) There is no such
thing as an "internationalized email user message"; the term applies
only to users and their agents and capabilities. In particular, the
use of non-ASCII, and hence presumably internationalized, message
content is an integral part of the MIME specifications [RFC2045] and
does not require these extensions (although it is compatible with
them).
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RFC 6530 Internationalized Email Framework February 20124.4. Messages
A "message" is sent from one user (the sender) using a particular
email address to one or more other recipient email addresses (often
referred to just as "users" or "recipient users").
4.5. Mailing Lists
A "mailing list" is a mechanism whereby a message may be distributed
to multiple recipients by sending it to one recipient address. An
agent (typically not a human being) at that single address then
causes the message to be redistributed to the target recipients.
This agent sets the envelope return address of the redistributed
message to a different address from that of the original single
recipient message. Using a different envelope return address
(reverse-path) causes error (and other automatically generated)
messages to go to an error-handling address.
Special provisions for managing mailing lists that might contain non-
ASCII addresses are discussed in a document that is specific to that
topic [RFC5983] and its expected successor [RFC5983bis-MailingList].
4.6. Conventional Message and Internationalized Message
o A conventional message is one that does not use any extension
defined in the SMTP extension document [RFC6531] or in the
UTF8header document [RFC6532] in this set of specifications, and
is strictly conformant to RFC 5322 [RFC5322].
o An internationalized message is a message utilizing one or more of
the extensions defined in this set of specifications, so that it
is no longer conformant to the traditional specification of an
email message or its transport.
4.7. Undeliverable Messages, Notification, and Delivery Receipts
As specified in RFC 5321, a message that is undeliverable for some
reason is expected to result in notification to the sender. This can
occur in either of two ways. One, typically called "Rejection",
occurs when an SMTP server returns a reply code indicating a fatal
error (a "5yz" code) or persistently returns a temporary failure
error (a "4yz" code). The other involves accepting the message
during SMTP processing and then generating a message to the sender,
typically known as a "Non-delivery Notification" or "NDN". Current
practice often favors rejection over NDNs because of the reduced
likelihood that the generation of NDNs will be used as a spamming
technique. The latter, NDN, case is unavoidable if an intermediate
MTA accepts a message that is then rejected by the next-hop server.
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RFC 6530 Internationalized Email Framework February 2012
A sender MAY also explicitly request message receipts [RFC3461] that
raise the same issues for these internationalization extensions as
NDNs.
5. Overview of the Approach and Document Plan
This set of specifications changes both SMTP and the character
encoding of email message headers to permit non-ASCII characters to
be represented directly. Each important component of the work is
described in a separate document. The document set, whose members
are described below, also contains Informational documents whose
purpose is to provide implementation suggestions and guidance for the
protocols.
In addition to this document, the following documents make up this
specification and provide advice and context for it.
o SMTP extension. The SMTP extension document [RFC6531] provides an
SMTP extension (as provided for in RFC 5321) for internationalized
addresses.
o Email message headers in UTF-8. The email message header document
[RFC6532] essentially updates RFC 5322 to permit some information
in email message headers to be expressed directly by Unicode
characters encoded in UTF-8 when the SMTP extension described
above is used. This document, possibly with one or more
supplemental ones, will also need to address the interactions with
MIME, including relationships between SMTPUTF8 and internal MIME
headers and content types.
o Extensions to delivery status and notification handling to adapt
to internationalized addresses [RFC6533].
o Forthcoming documents will specify extensions to the IMAP protocol
[RFC3501] to support internationalized message headers
[RFC5738bis-IMAP], parallel extensions to the POP protocol
[RFC5721] [RFC5721bis-POP3], and some common properties of the two
[POPIMAP-downgrade].
6. Review of Experimental Results
The key difference between this set of protocols and the experimental
set that preceded them [RFC5335] [RFC5336] [RFC5337] [RFC5504]
[RFC5721] [RFC5738] [RFC5825] is that the earlier group provided a
mechanism for in-transit downgrading of messages (described in detail
in RFC 5504). That mechanism permitted, and essentially required,
that each non-ASCII address be accompanied by an all-ASCII
equivalent. That, in turn, raised security concerns associated with
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RFC 6530 Internationalized Email Framework February 2012
pairing of addresses that could not be authenticated. It also
introduced the first incompatible change to Internet mail addressing
in many years, raising concerns about interoperability issues if the
new address forms "leaked" into legacy email implementations. After
examining experience with the earlier, experimental, predecessors of
these specifications, the working group that produced them concluded
that the advantages of in-transit downgrading, were it feasible
operationally, would be significant enough to overcome those
concerns.
That turned out not to be the case, with interoperability problems
among initial implementations. Prior to starting on the work that
led to this set of specifications, the WG concluded that the
combination of requirements and long-term implications of that
earlier model were too complex to be satisfactory and that work
should move ahead without it.
The other significant change to the protocols themselves is that the
SMTPUTF8 keyword is now required as an SMTP client announcement if
the extension is needed; in the experimental version, only the server
announcement that an extended envelope and/or content were permitted
was necessary.
7. Overview of Protocol Extensions and Changes7.1. SMTP Extension for Internationalized Email Address
An SMTP extension, "SMTPUTF8", is specified as follows:
o Permits the use of UTF-8 strings in email addresses, both local
parts and domain names.
o Permits the selective use of UTF-8 strings in email message
headers (see Section 7.2).
o Requires that the server advertise the 8BITMIME extension
[RFC6152] and that the client support 8-bit transmission so that
header information can be transmitted without using a special
content-transfer-encoding.
Some general principles affect the development decisions underlying
this work.
1. Email addresses enter subsystems (such as a user interface) that
may perform charset conversions or other encoding changes. When
the local part of the address includes characters outside the
ASCII character repertoire, use of ASCII-compatible encoding
(ACE) [RFC3492] [RFC5890] in the domain part is discouraged to
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RFC 6530 Internationalized Email Framework February 2012
promote consistent processing of characters throughout the
address.
2. An SMTP relay MUST
* Either recognize the format explicitly, agreeing to do so via
an ESMTP option, or
* Reject the message or, if necessary, return a non-delivery
notification message, so that the sender can make another
plan.
3. If the message cannot be forwarded because the next-hop system
cannot accept the extension, it MUST be rejected or a non-
delivery message MUST be generated and sent.
4. In the interest of interoperability, charsets other than UTF-8
are prohibited in mail addresses and message headers being
transmitted over the Internet. There is no practical way to
identify multiple charsets properly with an extension similar to
this without introducing great complexity.
Conformance to the group of standards specified here for email
transport and delivery requires implementation of the SMTP extension
specification and the UTF-8 header specification. If the system
implements IMAP or POP, it MUST conform to the internationalized IMAP
[RFC5738bis-IMAP] or POP [RFC5721bis-POP3] specifications
respectively.
7.2. Transmission of Email Header Fields in UTF-8 Encoding
There are many places in MUAs or in a user presentation in which
email addresses or domain names appear. Examples include the
conventional "From:", "To:", or "Cc:" header fields; "Message-ID:"
and "In-Reply-To:" header fields that normally contain domain names
(but that may be a special case); and in message bodies. Each of
these must be examined from an internationalization perspective. The
user will expect to see mailbox and domain names in local characters,
and to see them consistently. If non-obvious encodings, such as
protocol-specific ACE variants, are used, the user will inevitably,
if only occasionally, see them rather than "native" characters and
will find that discomfiting or astonishing. Similarly, if different
codings are used for mail transport and message bodies, the user is
particularly likely to be surprised, if only as a consequence of the
long-established "things leak" principle. The only practical way to
avoid these sources of discomfort, in both the medium and the longer
term, is to have the encodings used in transport be as similar to the
encodings used in message headers and message bodies as possible.
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RFC 6530 Internationalized Email Framework February 2012
When email local parts are internationalized, they SHOULD be
accompanied by arrangements for the message headers to be in the
fully internationalized form. That form SHOULD use UTF-8 rather than
ASCII as the base character set for the contents of header fields
(protocol elements such as the header field names themselves are
unchanged and remain entirely in ASCII). For transition purposes and
compatibility with legacy systems, this can be done by extending the
traditional MIME encoding models for non-ASCII characters in headers
[RFC2045] [RFC2231], but even these should be based on UTF-8, rather
than other encodings, if at all possible [RFC6055]. However, the
target is fully internationalized message headers, as discussed in
[RFC6532] and not an extended and painful transition.
7.3. SMTP Service Extension for DSNs
The existing Delivery Status Notifications (DSNs) specification
[RFC3461], which is a Draft Standard, is limited to ASCII text in the
machine-readable portions of the protocol. "International Delivery
and Disposition Notifications" [RFC6533] adds a new address type for
international email addresses so an original recipient address with
non-ASCII characters can be correctly preserved even after
downgrading. If an SMTP server advertises both the SMTPUTF8 and the
DSN extension, that server MUST implement internationalized DSNs
including support for the ORCPT parameter specified in RFC 3461
[RFC3461].
8. Downgrading before and after SMTP Transactions
An important issue with these extensions is how to handle
interactions between systems that support non-ASCII addresses and
legacy systems that expect ASCII. There is, of course, no problem
with ASCII-only systems sending to those that can handle
internationalized forms because the ASCII forms are just a proper
subset. But, when systems that support these extensions send mail,
they MAY include non-ASCII addresses for senders, receivers, or both
and might also provide non-ASCII header information other than
addresses. If the extension is not supported by the first-hop system
(i.e., the SMTP server accessed by the submission server acting as an
SMTP client), message-originating systems SHOULD be prepared to
either send conventional envelopes and message headers or to return
the message to the originating user so the message may be manually
downgraded to the traditional form, possibly using encoded words
[RFC2047] in the message headers. Of course, such transformations
imply that the originating user or system must have ASCII-only
addresses available for all senders and recipients. Mechanisms by
which such addresses may be found or identified are outside the scope
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RFC 6530 Internationalized Email Framework February 2012
of these specifications as are decisions about the design of
originating systems such as whether any required transformations are
made by the user, the originating MUA, or the submission server.
A somewhat more complex situation arises when the first-hop system
supports these extensions but some subsequent server in the SMTP
transmission chain does not. It is important to note that most cases
of that situation with forward-pointing addresses will be the result
of configuration errors: especially if it hosts non-ASCII addresses,
a final delivery MTA that accepts these extensions SHOULD NOT be
configured with lower-preference MX hosts that do not. When the only
non-ASCII address being transmitted is backward-pointing (e.g., in an
SMTP MAIL command), recipient configuration cannot help in general.
On the other hand, alternate, all-ASCII addresses for senders are
those most likely to be authoritatively known by the submission
environment or the sender herself. Consequently, if an intermediate
SMTP relay that requires these extensions then discovers that the
next system in the chain does not support them, it will have little
choice other than to reject or return the message.
As discussed above, downgrading to an ASCII-only form may occur
before or during the initial message submission. It might also occur
after the delivery to the final delivery MTA in order to accommodate
message stores, IMAP or POP servers, or clients that have different
capabilities than the delivery MTA. These cases are discussed in the
subsections below.
8.1. Downgrading before or during Message Submission
The IETF has traditionally avoided specifying the precise behavior of
MUAs to provide maximum flexibility in the associated user
interfaces. The SMTP standard [RFC5321], Section 6.4, gives wide
latitude to MUAs and submission servers as to what might be supplied
by the user as long as the result conforms with "on the wire"
standards once it is injected into the public Internet. In that
tradition, the discussion in the remainder of Section 8 is provided
as general guidance rather than normative requirements.
Messages that require these extensions will sometimes be transferred
to a system that does not support these extensions; it is likely that
the most common cases will involve the combination of ASCII-only
forward-pointing addresses with a non-ASCII backward-pointing one.
Until the extensions described here have been universally implemented
in the Internet email environment, senders who prefer to use non-
ASCII addresses (or raw UTF-8 characters in header fields), even when
their intended recipients use and expect all-ASCII ones, will need to
be especially careful about the error conditions that can arise. The
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risks are especially great in environments in which non-delivery
messages (or other indications from submission servers) are routinely
dropped or ignored.
Perhaps obviously, the most convenient time to find an ASCII address
corresponding to an internationalized address is at the originating
MUA or closely associated systems. This can occur either before the
message is sent or after the internationalized form of the message is
rejected. It is also the most convenient time to convert a message
from the internationalized form into conventional ASCII form or to
generate a non-delivery message to the sender if either is necessary.
At that point, the user has a full range of choices available,
including changing backward-pointing addresses, contacting the
intended recipient out of band for an alternate address, consulting
appropriate directories, arranging for translation of both addresses
and message content into a different language, and so on. While it
is natural to think of message downgrading as optimally being a fully
automated process, we should not underestimate the capabilities of a
user of at least moderate intelligence who wishes to communicate with
another such user.
In this context, one can easily imagine modifications to message
submission servers (as described in RFC 6409 [RFC6409]) so that they
would perform downgrading operations or perhaps even upgrading ones.
Such operations would permit receiving messages with one or more of
the internationalization extensions discussed here and adapting the
outgoing message, as needed, to respond to the delivery or next-hop
environment the submission server encounters.
8.2. Downgrading or Other Processing after Final SMTP Delivery
When an email message is received by a final delivery MTA, it is
usually stored in some form. Then it is retrieved either by software
that reads the stored form directly or by client software via some
email retrieval mechanisms such as POP or IMAP.
The SMTP extension described in Section 7.1 provides protection only
in transport. It does not prevent MUAs and email retrieval
mechanisms that have not been upgraded to understand
internationalized addresses and UTF-8 message headers from accessing
stored internationalized emails.
Since the final delivery MTA (or, to be more specific, its
corresponding mail storage agent) cannot safely assume that agents
accessing email storage will always be capable of handling the
extensions proposed here, it MAY downgrade internationalized emails,
specially identify messages that utilize these extensions, or both.
If either or both of these actions were to be taken, the final
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delivery MTA SHOULD include a mechanism to preserve or recover the
original internationalized forms without information loss.
Preservation of that information is necessary to support access by
SMTPUTF8-aware agents.
9. Downgrading in Transit
The base SMTP specification (Section 2.3.11 of RFC 5321 [RFC5321])
states that "due to a long history of problems when intermediate
hosts have attempted to optimize transport by modifying them, the
local-part MUST be interpreted and assigned semantics only by the
host specified in the domain part of the address". This is not a new
requirement; equivalent statements appeared in specifications in 2001
[RFC2821] and even in 1989 [RFC1123].
Adherence to this rule means that a downgrade mechanism that
transforms the local part of an email address cannot be utilized in
transit. It can only be applied at the endpoints, specifically by
the MUA or submission server or by the final delivery MTA.
One of the reasons for this rule has to do with legacy email systems
that embed mail routing information in the local part of the address
field. Transforming the email address destroys such routing
information. There is no way a server other than the final delivery
server can know, for example, whether the local part of
user%foo@example.com is a route ("user" is reached via "foo") or
simply a local address.
10. User Interface and Configuration Issues
Internationalization of addresses and message headers, especially in
combination with variations on character coding that are inherent to
Unicode, may make careful choices of addresses and careful
configuration of servers and DNS records even more important than
they are for traditional Internet email. It is likely that, as
experience develops with the use of these protocols, it will be
desirable to produce one or more additional documents that offer
guidance for configuration and interfaces. A document that discusses
issues with MUAs, especially with regard to downgrading, is expected
to be developed. The subsections below address some other issues.
10.1. Choices of Mailbox Names and Unicode Normalization
It has long been the case that the email syntax permits choices about
mailbox names that are unwise in practice, if one actually intends
the mailboxes to be accessible to a broad range of senders. The most
often cited examples involve the use of case-sensitivity and tricky
quoting of embedded characters in mailbox local parts. These
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deliberately unusual constructions are permitted by the protocols,
and servers are expected to support them. Although they can provide
value in special cases, taking advantage of them is almost always bad
practice unless the intent is to create some form of security by
obscurity.
In the absence of these extensions, SMTP clients and servers are
constrained to using only those addresses permitted by RFC 5321. The
local parts of those addresses MAY be made up of any ASCII characters
except the control characters that RFC 5321 prohibits, although some
of them MUST be quoted as specified there. It is notable in an
internationalization context that there is a long history on some
systems of using overstruck ASCII characters (a character, a
backspace, and another character) within a quoted string to
approximate non-ASCII characters. This form of internationalization
was permitted by RFC 821 [RFC0821] but is prohibited by RFC 5321
because it requires a backspace character (a prohibited C0 control).
Because RFC 5321 (and its predecessor, RFC 2821) prohibit the use of
this character in ASCII mailbox names and it is even more problematic
(for canonicalization and normalization reasons) in non-ASCII
strings, backspace MUST NOT appear in SMTPUTF8 mailbox names.
For the particular case of mailbox names that contain non-ASCII
characters in the local part, domain part, or both, special attention
MUST be paid to Unicode normalization [Unicode-UAX15], in part
because Unicode strings may be normalized by other processes
independent of what a mail protocol specifies (this is exactly
analogous to what may happen with quoting and dequoting in
traditional addresses). Consequently, the following principles are
offered as advice to those who are selecting names for mailboxes:
o In general, it is wise to support addresses in Normalized form,
using at least Normalization Form NFC. Except in circumstances in
which NFKC would map characters together that the parties
responsible for the destination mail server would prefer to be
kept distinguishable, supporting the NFKC-conformant form would
yield even more predictable behavior for the typical user.
o It will usually be wise to support other forms of the same local-
part string, either as aliases or by normalization of strings
reaching the delivery server: the sender should not be depended
upon to send the strings in normalized form.
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o Stated differently and in more specific terms, the rules of the
protocol for local-part strings essentially provide that:
* Unnormalized strings are valid, but sufficiently bad practice
that they may not work reliably on a global basis. Servers
should not depend on clients to send normalized forms but
should be aware that procedures on client machines outside the
control of the MUA may cause normalized strings to be sent
regardless of user intent.
* C0 (and presumably C1) controls (see The Unicode Standard
[Unicode]) are prohibited, the first in RFC 5321 and the second
by an obvious extension from it [RFC5198].
* Other kinds of punctuation, spaces, etc., are risky practice.
Perhaps they will work, and SMTP receiver code is required to
handle them without severe errors (even if such strings are not
accepted in addresses to be delivered on that server), but
creating dependencies on them in mailbox names that are chosen
is usually a bad practice and may lead to interoperability
problems.
11. Additional Issues
This section identifies issues that are not covered, or not covered
comprehensively, as part of this set of specifications, but that will
require ongoing review as part of deployment of email address and
header internationalization.
11.1. Impact on URIs and IRIs
The mailto: schema [RFC6068], and the discussion of it in the
Internationalized Resource Identifier (IRI) specification [RFC3987],
may need to be modified when this work is completed and standardized.
11.2. Use of Email Addresses as Identifiers
There are a number of places in contemporary Internet usage in which
email addresses are used as identifiers for individuals, including as
identifiers to Web servers supporting some electronic commerce sites
and in some X.509 certificates [RFC5280]. These documents do not
address those uses, but it is reasonable to expect that some
difficulties will be encountered when internationalized addresses are
first used in those contexts, many of which cannot even handle the
full range of addresses permitted today.
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RFC 6530 Internationalized Email Framework February 201211.3. Encoded Words, Signed Messages, and Downgrading
One particular characteristic of the email format is its persistency:
MUAs are expected to handle messages that were originally sent
decades ago and not just those delivered seconds ago. As such, MUAs
and mail filtering software, such as that specified in Sieve
[RFC5228], will need to continue to accept and decode header fields
that use the "encoded word" mechanism [RFC2047] to accommodate non-
ASCII characters in some header fields. While extensions to both
POP3 [RFC1939] and IMAP [RFC3501] have been defined that include
automatic upgrading of messages that carry non-ASCII information in
encoded form -- including RFC 2047 decoding -- of messages by the
POP3 [RFC5721bis-POP3] or IMAP [RFC5738bis-IMAP] server, there are
message structures and MIME content-types for which that cannot be
done or where the change would have unacceptable side effects.
For example, message parts that are cryptographically signed, using
e.g., S/MIME [RFC5751] or Pretty Good Privacy (PGP) [RFC3156], cannot
be upgraded from the RFC 2047 form to normal UTF-8 characters without
breaking the signature. Similarly, message parts that are encrypted
may contain, when decrypted, header fields that use the RFC 2047
encoding; such messages cannot be 'fully' upgraded without access to
cryptographic keys.
Similar issues may arise if messages are signed and then subsequently
downgraded, e.g., as discussed in Section 8.1, and then an attempt is
made to upgrade them to the original form and then verify the
signatures. Even the very subtle changes that may result from
algorithms to downgrade and then upgrade again may be sufficient to
invalidate the signatures if they impact either the primary or MIME
body part headers. When signatures are present, downgrading must be
performed with extreme care if at all.
11.4. Other Uses of Local Parts
Local parts are sometimes used to construct domain labels, e.g., the
local part "user" in the address user@domain.example could be
converted into a host name user.domain.example with its Web space at
<http://user.domain.example> and the catch-all addresses
any.thing.goes@user.domain.example.
Such schemes are obviously limited by, among other things, the SMTP
rules for domain names, and will not work without further
restrictions for other local parts. Whether those limitations are
relevant to these specifications is an open question. It may be
simply another case of the considerable flexibility accorded to
delivery MTAs in determining the mailbox names they will accept and
how they are interpreted.
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RFC 6530 Internationalized Email Framework February 201211.5. Non-Standard Encapsulation Formats
Some applications use formats similar to the application/mbox format
[RFC4155] instead of the message/digest form defined in RFC 2046,
Section 5.1.5 [RFC2046] to transfer multiple messages as single
units. Insofar as such applications assume that all stored messages
use the message/rfc822 format described in RFC 2046, Section 5.2.1
[RFC2046] with ASCII message headers, they are not ready for the
extensions specified in this series of documents, and special
measures may be needed to properly detect and process them.
12. Key Changes from the Experimental Protocols and Framework
The original framework for internationalized email addresses and
headers was described in RFC 4952 and a subsequent set of
experimental protocol documents. Those relationships are described
in Section 3. The key architectural difference between the
experimental specifications and this newer set is that the earlier
specifications supported in-transit downgrading. Those mechanisms
included the definition of syntax and functions to support passing
alternate, all-ASCII addresses with the non-ASCII ones as well as
special headers to indicate the downgraded status of messages. Those
features were eliminated after experimentation indicated that they
were more complex and less necessary than had been assumed earlier.
Those issues are described in more detail in Sections 6 and 9.
13. Security Considerations
Any expansion of permitted characters and encoding forms in email
addresses raises some risks. There have been discussions on so
called "IDN-spoofing" or "IDN homograph attacks". These attacks
allow an attacker (or "phisher") to spoof the domain or URLs of
businesses or other entities. The same kind of attack is also
possible on the local part of internationalized email addresses. It
should be noted that the proposed fix involving forcing all displayed
elements into normalized lowercase works for domain names in URLs,
but not for email local parts since those are case sensitive.
Since email addresses are often transcribed from business cards and
notes on paper, they are subject to problems arising from confusable
characters (see [RFC4690]). These problems are somewhat reduced if
the domain associated with the mailbox is unambiguous and supports a
relatively small number of mailboxes whose names follow local system
conventions. They are increased with very large mail systems in
which users can freely select their own addresses.
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The internationalization of email addresses and message headers must
not leave the Internet less secure than it is without the required
extensions. The requirements and mechanisms documented in this set
of specifications do not, in general, raise any new security issues.
They do require a review of issues associated with confusable
characters -- a topic that is being explored thoroughly elsewhere
(see, e.g., RFC 4690 [RFC4690]) -- and, potentially, some issues with
UTF-8 normalization, discussed in RFC 3629 [RFC3629], and other
transformations. Normalization and other issues associated with
transformations and standard forms are also part of the subject of
work described elsewhere [RFC5198] [RFC5893] [RFC6055].
Some issues specifically related to internationalized addresses and
message headers are discussed in more detail in the other documents
in this set. However, in particular, caution should be taken that
any "downgrading" mechanism, or use of downgraded addresses, does not
inappropriately assume authenticated bindings between the
internationalized and ASCII addresses. This potential problem can be
mitigated somewhat by enforcing the expectation that most or all such
transformations will be performed prior to final delivery by systems
that are presumed to be under the administrative control of the
sending user (as opposed to being performed in transit by entities
that are not under the administrative control of the sending user).
The new UTF-8 header and message formats might also raise, or
aggravate, another known issue. If the model creates new forms of an
'invalid' or 'malformed' message, then a new email attack is created:
in an effort to be robust, some or most agents will accept such
messages and interpret them as if they were well-formed. If a filter
interprets such a message differently than the MUA used by the
recipient, then it may be possible to create a message that appears
acceptable under the filter's interpretation but that should be
rejected under the interpretation given to it by that MUA. Such
attacks already have occurred for existing messages and encoding
layers, e.g., invalid MIME syntax, invalid HTML markup, and invalid
coding of particular image types.
In addition, email addresses are used in many contexts other than
sending mail, such as for identifiers under various circumstances
(see Section 11.2). Each of those contexts will need to be
evaluated, in turn, to determine whether the use of non-ASCII forms
is appropriate and what particular issues they raise.
This work will clearly affect any systems or mechanisms that are
dependent on digital signatures or similar integrity protection for
email message headers (see also the discussion in Section 11.3).
Many conventional uses of PGP and S/MIME are not affected since they
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are used to sign body parts but not message headers. On the other
hand, the developing work on DomainKeys Identified Mail (DKIM)
[RFC5863] will eventually need to consider this work, and vice versa:
while this specification does not address or solve the issues raised
by DKIM and other signed header mechanisms, the issues will have to
be coordinated and resolved eventually if the two sets of protocols
are to coexist. In addition, to the degree to which email addresses
appear in PKI (Public Key Infrastructure) certificates [RFC5280],
standards addressing such certificates will need to be upgraded to
address these internationalized addresses. Those upgrades will need
to address questions of spoofing by look-alikes of the addresses
themselves.
14. Acknowledgments
This document is an update to, and derived from, RFC 4952. This
document would have been impossible without the work and
contributions acknowledged in it. The present document benefited
significantly from discussions in the IETF EAI working group and
elsewhere after RFC 4952 was published, especially discussions about
the experimental versions of other documents in the internationalized
email collection, and from RFC errata on RFC 4952 itself.
Special thanks are due to Ernie Dainow for careful reviews and
suggested text in this version and to several IESG members for a
careful review and specific suggestions.
15. References15.1. Normative References
[ASCII] American National Standards Institute (formerly United
States of America Standards Institute), "USA Code for
Information Interchange", ANSI X3.4-1968, 1968.
ANSI X3.4-1968 has been replaced by newer versions with
slight modifications, but the 1968 version remains
definitive for the Internet.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
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